There is an increasing need for resins with good dimensional stability (low coefficient of thermal expansion (CTE), high glass transition temperature Tg and high modulus) and moisture and heat resistance over a wide temperature range. There is a particular need for resins which can be applied in a curable state and which can be cured in a thick section and are thus suitable for encapsulating delicate substrates, for example as underfill for microelectronic device packaging, as matrix resin in composites, and also in coatings such as wafer level and solar panel coatings, in planarization layers for Flat Panel Displays and in photonic devices.
Silicone resins have excellent heat resistance and are moisture repellent but typically have a CTE in the range 110 to 350 ppm/° C., compared to 50 to 120 ppm/° C. for most organic polymers and resins. The present invention seeks to produce silicone resins having reduce CTE in the cured state so that they are more suitable for the uses listed above
U.S. Pat. No. 6,124,407 describes a silicone composition comprising (A) 100 parts by weight of a polydiorganosiloxane containing an average of at least two silicon-bonded alkenyl groups per molecule; (B) 75 to 150 parts by weight of an organopolysiloxane resin containing an average of from 2.5 to 7.5 mole percent of alkenyl groups; (C) an organohydrogenpolysiloxane having an average of at least three silicon-bonded hydrogen atoms per molecule in an amount to provide from one to three silicon-bonded hydrogen atoms per alkenyl group in components (A) and (B) combined; (D) an adhesion promoter in an amount to effect adhesion of the composition to a substrate; and (E) a hydrosilylation catalyst in an amount to cure the composition. The composition is useful as an encapsulant in chip scale packages.
Siloxane resins containing at least two alkenyl groups, present for example as dimethylvinylsilyl units, are described in U.S. Pat. No. 5,777,047 and U.S. Pat. No. 5,468,828. These siloxane resins are used in minor amounts to cure a polydiorganosilane to an elastomer, rather than to form a rigid thermoset resin.